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Emplacement temperature and flow direction analysis of large dimension calderas ignimbrites: the Cerro Galan and Toconquis group ignimbrites (Puna plateau, NW Argentina)http://hdl.handle.net/2307/599
<Title>Emplacement temperature and flow direction analysis of large dimension calderas ignimbrites: the Cerro Galan and Toconquis group ignimbrites (Puna plateau, NW Argentina)</Title>
<Authors>Lesti, Chiara</Authors>
<Issue Date>2010-03-26</Issue Date>
<Abstract>Estimates of pyroclastic flow emplacement temperatures in the Cerro Galàn ignimbrite and
Toconquis Group ignimbrites and flow directions were determined using thermal remanent
magnetization (TRM) of lithic clasts embedded within the deposits and the anisotropy of magnetic
susceptibility, respectively. These ignimbrites belong to the Cerro Galàn volcanic system, one of the
largest calderas in the world, in the Puna plateau, NW Argentina.
Temperature estimates for the 2.16 ± 0.16 Ma Cerro Galán ignimbrite are retrieved from 40 sites
in 14 localities (176 measured clasts), distributed at different distances from the caldera and
different stratigraphic heights. Additionally, temperature estimates were obtained from 27 sample
sites (125 measured clasts) in seven ignimbrite units forming the older Toconquis Group (5.60±0.20
- 4.51±0.11 Ma), mainly outcropping along a type-section at Rio Las Pitas, Vega Real Grande.
Flow directions are defined in 35 sites (403 measures specimens), distributed again at different
distances and azimuth from the caldera and at different stratigraphic heights along the same section.
The flow direction assessment was carried out on the Cerro Galàn ignimbrite and not on the older
units, as the outcrops of the Toconquis Group are limited in extension mainly to the west of the
caldera.
The paleomagnetic data obtained by progressive thermal demagnetization (PTD), show that most
of the clasts of the Cerro Galán ignimbrite have one single magnetic component, oriented close to
the expected geomagnetic field at the time of emplacement. Results show therefore that most of the
clasts acquired a new magnetization oriented parallel to the magnetic field at the moment of the
ignimbrite deposition, suggesting that the clasts were heated up to or above the highest blocking
temperature (Tb) of the magnetic minerals (Tb=580°C for magnetite; Tb=600-630°C for titanohematite).
We obtained similar emplacement temperature estimations for five out of six volcanic units
belonging to the Toconquis Group, with the exception of one unit (Lower Merihuaca), where we
found two distinct magnetic components. The estimation of emplacement temperatures in this latter
case is constrained between 580°-610°C.
The study of the AMS was performed in order to analyse the ignimbrite fabric and to evaluate
the flow structure and emplacement mechanism, relating the magnetic fabric to the
paleotopography.
Flow directions are defined in 35 sites (403 measures specimens), distributed at different
distances and azimuth from the caldera and at different stratigraphic heights along the same section.
The comparison of magnetic fabric with mineralogical fabric is also examined to assess the
reliability of the AMS as flow direction indicator.
AMS results show a strong uniformity throughout the ignimbrite, with the exception of sites
where the topographic control on the emplacement mechanism is higher. Flow directions results
show a radial pattern around the caldera, in proximal sites, while in distal sites the directions are
deflected by the paleotopography.
The strong control of the paleotopography revealed in this study, together with field evidences of
low level of turbulence and high emplacement temperatures estimation found, indicate that the flow
was highly concentrated throughout the flow path.
We conclude that the Cerro Galán ignimbrite and Toconquis Group ignimbrites were emplaced
at temperatures equal to or higher than 620°C and that in distal sites the flow, besides having the
capacity of travel up to 80 Km, follows passively the paleotopography. The homogeneity of high
temperatures from proximal to distal facies, and the behaviour in presence of topographic obstacles
in distal sites, provide constraints for an emplacement model for the Cerro Galán ignimbrite,
marked by a relatively low eruption column, low levels of turbulence during deposition, air
entrainment, surface-water interaction, and a high level of topographic confinement, all ensuring
minimal heat loss and high concentration flow.</Abstract>Thu, 25 Mar 2010 23:00:00 GMThttp://hdl.handle.net/2307/5992010-03-25T23:00:00Z